1. Field of the Invention
This invention relates to hot rolled steel sheets that are suitably useful for automotive vehicles, household appliances, mechanical structures and constructional materials. More particularly, it relates to such a hot rolled steel sheet which is ultrafine in grain structure as hot-rolled and does not need extra heat treatment, highly ductile and tough, and superior in the strength-elongation balance, and further, is less anisotropic with regard to the mechanical characteristics, particularly ductility.
The term "ultrafine grain structure" as used herein denotes a crystal structure composed of a main phase (usually a ferrite phase), the average crystal grain size (hereinafter called the "average grain size") of which is less than about 4 .mu.m
2. Description of the Related Art
Steel materials to be used for automotive vehicles, household appliances, mechanical structures and constructional materials are required to be superior in mechanical properties, such as strength, formability and toughness. Structural fine grains are advantageous as being capable of improving the above mechanical properties as a whole. Thus, a number of methods have been proposed for producing steel materials with fine grain structures.
As regards high tensile steel, the focus of attention has recently been directed to the development of a high tensile steel sheet which could provide a proper balance between low costs and high functional characteristics. Moreover, a steel sheet for use in automobiles needs superior impact resistance, in addition to high mechanical strength, so as to keep the passengers safe in case of collision of a car. Importantly, therefore, high tensile steel should be brought into a finely grained structure to prevent the same from becoming deteriorated in respect of ductility, toughness and fatigue ratio when steel is made highly tensile.
As means for producing fine grain structures, there are known large-reduction rolling, controlled rolling and controlled cooling.
Large-reduction rolling is disclosed typically by Japanese Unexamined Patent Publication No. 58-123823 and Japanese Examined Patent Publication No. 5-65564, for example. The mechanisms of structural fine graining found in both of these publications contemplate applying large reduction to austenite grains so that the strain-induced .gamma. to .alpha. a transformation is accelerated. These methods are capable of achieving fine grain structures to some extent, but are defective in that they are difficult to be made feasible by means of a hot strip mill in common use because a hot reduction of not less than 40% is necessary per pass. As another problem, the resultant mechanical properties are caused to be anisotropic because the grains are flattened due to large-reduction rolling, or the absorption of fracture energy is reduced due to grain separation.
An example resulting from use of controlled rolling and controlled cooling is a precipitation strengthened steel sheet containing Nb or Ti. This steel sheet is obtained by being made highly tensile with the utilization of precipitation strengthening by Nb or Ti and by being finish-rolled at low temperature utilizing recrystallization prevention in austenite grains provided from Nb or Ti, resulting in fine ferrite grains by the strain-induced .gamma. to .alpha. transformation from non-recrystallized deformed austenite grains. However, such a steel sheet has the problem that the mechanical properties are greatly anisotropic. With regard to a steel sheet to be used for automobiles and subjected to press forming, for example, the criticality of formability is determined by the level of characteristics in the least elongated direction of the steel sheet. Thus, a greatly anisotropic steel sheet can never produce the characteristic effects of structural fine grains in some instances. Similar reasoning applies also to mechanical structures; that is, an anisotropic steel sheet causes toughness and fatigue strength to be greatly anisotropic, and both of these mechanical properties are important to such a mechanical structure. Consequently, this often fails to exhibit the characteristics of structural fine grains.
In Japanese Unexamined Patent Publication No. 2-301540, a steel structure is disclosed which is composed chiefly of isotropic ferrite grains having an average grain size of not more than 5 .mu.m. Such steel structure is made by preparing a starting steel material having ferrite at at least one portion of the steel, by heating the steel material, while adding plastic deformation, to a temperature region not less than the critical point (Ac.sub.1 point), or by retaining the steel material in a temperature range of not less than the Ac.sub.1 point for a certain time subsequently to the above heating so that the steel material is structurally reverse-transformed in part or wholly into austenite, to provide ultrafine austenite grains, and thereafter by cooling the steel material thus treated. In this publication, the ferrite grains formed from transformed austenite are termed the isotropic ferrite grains to be distinguished from non-isotropic ferrite, such as pearlite, bainite or martensite. However, anisotropy cannot be eliminated even by use of this conventional method.
Recently, structural fine graining has been performed by allowing austenite grains to be extremely fine prior to hot rolling, followed by rolling and by structural fine graining with the use of dynamic recrystallization and controlled cooling. Exemplary methods are disclosed, for example, in Japanese Unexamined Patent Publications Nos. 9-87798, 9-143570 and 10-8138.
Japanese Unexamined Patent Publication No. 9-87798 discloses a method of producing a high-tensile hot-rolled steel sheet containing not less than 75% by volume of polygonal ferrite having an average grain size of less than 10 .mu.m and 5 to 20% by volume of residual austenite. This method comprises: heating a slab at 950 to 1100.degree. C., the slab containing 1.0 to 2.5% by weight of Mn, or not more than 2.5% by weight of Mn, and 0.05 to 0.30% by weight of Ti, or 0.05 to 0.30% by weight of Ti and not more than 0.30% by weight of Nb; hot-rolling the slab at least twice at a reduction of not less than 20% per pass; hot-rolling the slab at a finish-rolling temperature of not lower than the Ar.sub.3 transformation temperature; cooling the hot-rolled steel strip at a cooling speed of not less than 20.degree. C./sec; and coiling the resultant steel strip at 350 to 550.degree. C. to obtain the desired steel sheet.
Japanese Unexamined Patent Publication No. 9-143570 discloses a method of producing a high-tensile hot-rolled steel sheet containing not less than 80% by volume of ferrite having an average grain size of less than 10 .mu.m. This method comprises: heating steel at 950 to 1100.degree. C., the slab containing either one or both of 0.05 to 0.3% by weight of Ti and not more than 0.10% by weight of Nb; hot-rolling the steel at least twice at a reduction of not less than 20% per pass; hot-rolling the steel at a finish-rolling temperature of not lower than the Ar.sub.3 transformation temperature; cooling the hot-rolled steel strip at a cooling speed of not less than 20.degree. C./sec at from the Ar.sub.3 point to 750.degree. C.; retaining the cooled steel strip in a temperature range of lower than 750.degree. C. to 600.degree. C. for 5 to 20 seconds, and once again cooling the hot steel strip to a temperature of not higher than 550.degree. C. at a cooling speed of not less than 20.degree. C./sec; and coiling the resultant steel strip at a temperature of not higher than 550.degree. C. to obtain the desired steel sheet.
Japanese Unexamined Patent Publication No. 10-8138 discloses a method of producing a high-tensile hot-rolled steel sheet containing ferrite and residual austenite. This method comprises: heating a slab at 950 to 1100.degree. C., the slab containing not more than 1.0% by weight of Mn and 0.05 to 0.30% by weight of Ti, or Nb replaced partly or wholly by Ti and in an amount of twice that of Ti; hot-rolling the slab at least twice at a reduction of not less than 20% per pass; hot-rolling the slab at a finish-rolling temperature of not lower than the Ar.sub.3 transformation temperature; cooling the hot-rolled steel strip at a cooling speed of not less than 20.degree. C./sec; and coiling the resultant steel strip at 350 to 550.degree. C. to obtain the desired steel sheet.
The techniques disclosed in Japanese Unexamined Patent Publications Nos. 9-87798, 9-143570 and 10-8138 aim principally at providing steel sheets having fine-grained structures. Such a technique gives a steel sheet having an average grain size of approximately 3.6 .mu.m and having improved strength and ductility. However, this steel sheet is not acceptable with respect to the anisotropy of its mechanical characteristics, and particularly formability when it is applied to automobiles, and hence, is required to be much less anisotropic.
Consequently, a need exists for a hot rolled steel sheet having an ultrafine grain structure, reduced anisotropy and high formability.